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Replace a Transformer With a Battery

Discussion in 'Power Electronics' started by SaintApoc, May 1, 2017.

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  1. SaintApoc

    SaintApoc

    15
    0
    May 1, 2017
    I was not sure whether to post here or the general chat, so I apologize in advance.

    I have a computer speaker set with a subwoofer and the board's supply is a transformer. Its input is 120v AC (standard house voltage) and output 14.4v 10A DC.

    My hope is to replace the transformer with a battery/group of cells that I can recharge such that the circuit can either run from the wall or off of the batteries as well as be able to recharge the batteries. I would think running continuous at peak power for two hours is reasonable (though I would really like four).

    I want to use lithium ion or lithium polymer batteries, but even lead acid will get the job done. The problem is I just don't know what battery or cells to use, if they need a protection circuit (and what model if they do), and what circuit to use to allow the board to run from either batteries when unplugged or from the wall when plugged AND charge the batteries. I suspect the boards exist I just don't know the terminology to search for them or what the exact model I would need is.

    TLDR: Where do I get/how do I make:
    1. 14.4v 10A DC rechargeable battery running at least two hours at continuous peak
    2. if necessary, a protection circuit for 1.
    3. an automatic switch between outlet (120v AC) and battery that also charges the battery when plugged in.

    It is possible that if at least I have 1. taken care of I could devise a more manual way of doing things (parallel lines with a switch to change from wall to battery - maybe throw in some diodes where necessary. then just remove the battery and charge it in a standalone charger).

    Thanks in advance!
    -Craig
     
  2. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    A 12V lead acid battery will be easier to deal with than a lithium battery. Just charge it to 14V and take output from the battery. Remember to use a fuse for safety, batteries can put out very high currents.

    Batteries which are used in electric scooters or golf buggies would be suitable. You will need at least 20Ah if run at 10A but an audio amplifier when playing music will not be taking maximum current contiuously.
     
  3. SaintApoc

    SaintApoc

    15
    0
    May 1, 2017
    Thank you for the reply!
    Ok, so let me make sure the logic is correct in my head.
    I get a lead acid battery of suitable rating, and get a lead acid battery charger. I can put in a relay so that anytime the amplifier is plugged in, while sending power to both the transformer and the battery charger, it will also close the circuit from the transformer's output side and open the circuit from the battery.

    If I understand switches properly, closing "line 1" on a single pole double throw switch will open "line 2," and closing "line 2" opens "line 1." If this is the case, then does there exist a relay which closes one line and opens another based on power or no power (I am assuming this relay would have 5 pins/leads)? I am going to search for this right now.

    I hope my logic is sound - let me know if I am not thinking properly. Thank you again!
    -Craig
     
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The burgler alarms which I have seen have had a lead acid battery connected to the detector system and a small 1A charger connected to the battery, set to 13.8 or 14V. Thus when power is demanded it comes from the battery and the battery is recharged over a considerable time when mains is restored. No switching is involved.

    I have a handheld transceiver with a lithium battery which expired when I went into hospital and it was not used for a few months.
     
  5. SaintApoc

    SaintApoc

    15
    0
    May 1, 2017
    Oh that's great! thank you again for your help! Now to find a burglar alarm :)
     
  6. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Just break a few windows, you will soon find one:)
    They come up every now and again at my radio club junk sale. The batteries are often dead. The charger is usually a low voltage transformer driving a LM317T regulator. I have combined two to provide 24V for an antique railway station clock.
     
  7. SaintApoc

    SaintApoc

    15
    0
    May 1, 2017
    So, the plot thickens.
    Despite the transformer clearly stating output is 14.4VDC, my multimeter will only read it on the AC setting, and the pcb will only function if I rapidly tap and remove the makeshift battery I have, no matter polarity.

    This leads me to believe the transformer is putting out AC and not DC. Guess I need an inverter :/
     
  8. ChosunOne

    ChosunOne

    432
    132
    Jun 20, 2010
    If you can use an alarm system control panel's nominal 12 VDC output (which actually operates at closer to 13.5 VDC unless it's overloaded) with your inverter, then talk to me. I have a couple of useless-but-functional panels around somewhere that might work for you, and I can label where to connect your existing AC transformer to power the panel, and where to connect the DC output. I recommend buying a 12 VDC, 4 or 7 AH sealed lead-acid (SLA) battery to go with it, depending on how big a battery you need.

    I don't know what Amperage your system draws while operating on battery, so you'll have to do the math yourself. If you want your battery to last many years, find what Amperage is being drawn, and make your best guess at the longest time you'll have to operate on the battery alone. Calculate the Amphere-hours, and multiply by 4 for your standby battery capacity.

    Do NOT count on your battery lasting long if you discharge it over 90% of its capacity more than a time or two (ideally never). To get the maximum cycle life from your battery, try not to discharge it below 25% of capacity. Alarm system SLA batteries typically last about 5 years, but that's predicated on designing the system with a 12-hour backup time and power outages that rarely exceed 3 hours.
     
    duke37 likes this.
  9. SaintApoc

    SaintApoc

    15
    0
    May 1, 2017
    sorry for the late reply - been busy with other things. As it stands I have already given the item away in its current condition. I may do something like this again some day so thank you for your support!
    -Craig
     
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